Remove invalid fallback revalidate value (#69990)
This removes our invalid revalidate default fallback when a revalidate
value isn't returned from render as render should always return a valid
value. We also already have an invariant when an invalid revalidate
value is returned we just need to ensure that case could be hit
properly. This also fixes some invalid test cases which were sending
invalid headers.
x-ref: [slack
thread](https://vercel.slack.com/archives/C0676QZBWKS/p1726061828198529?thread_ts=1720714625.621179&cid=C0676QZBWKS)
